I would prefer that instead of cases, blank cards would drop. Just with their rarity. No merc, no weapons, no augments on them.
I dont like having 5 leads and 2 irons on screen when I pick my merc & load out. That way you could just let the lower tier cards stack until you go on a trade up spree. If you get a silver, gold, or what ever you are interested in, you can roll them immediatly.
At the moment, I have to wait till I get three leads or iron and trade them up to the next higher tier of a merc I usually dont play, just to keep some order.